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An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| High Excitation Molecular Gas in the Magellanic Clouds
We present the first survey of submillimeter 12CO J=4-->3emission in the Magellanic Clouds. The survey is comprised of 156'×6' maps obtained using the AST/ROtelescope toward the molecular peaks of the Large and Small MagellanicClouds. We have used these data to constrain the physical conditions inthese objects, in particular, their molecular gas density andtemperature. We find that there are significant amounts of warmmolecular gas associated with most of these molecular peaks and thathigh molecular gas temperatures are pervasive throughout our sample. Wediscuss whether this may be due to the low metallicities and theassociated dearth of gas coolants in the Clouds and conclude that thepresent sample is insufficient to assert this effect.
| A New Isotopic Abundance Anomaly in Chemically Peculiar Stars
Not Available
| The LMC H II region N 214C and its peculiar nebular blob
We study the Large Magellanic Cloud H ii region N 214C using imaging andspectroscopy obtained at the ESO New Technology Telescope. On the basisof the highest resolution images so far obtained of the OB associationLH 110, we show that the main exciting source of the H ii region, Sk-71°51, is in fact a tight cluster of massive stars consisting of atleast 6 components in an area ~4'' wide. Spectroscopic observationsallow us to revise the spectral type of the main component (# 17) to O2V ((f*)) + OB, a very rare, hot type. We also classify several otherstars associated with N 214C and study the extinction and excitationcharacteristics of the H ii region. Moreover, we obtain BVR photometryand astrometry of 2365 stars and from the corresponding color-magnitudediagram study the stellar content of N 214C and the surrounding LH 110.Furthermore, we discover a striking compact blob of ionized gas in theouter northern part of N 214C. A spherical structure of ~5'' in radius(~1.3 pc), it is split into two lobes by absorbing dust runningdiametrically through its center. We discuss the possible nature of thisobject.
| Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems
The method developed by Gouliermis et al. (\cite{Gouliermis00}, PaperI), for the detection and classification of stellar systems in the LMC,was used for the identification of stellar associations and openclusters in the central area of the LMC. This method was applied on thestellar catalog produced from a scanned 1.2 m UK Schmidt Telescope Platein U with a field of view almost 6\fdg5 x 6\fdg5, centered on the Bar ofthis galaxy. The survey of the identified systems is presented herefollowed by the results of the investigation on their spatialdistribution and their structural parameters, as were estimatedaccording to our proposed methodology in Paper I. The detected openclusters and stellar associations show to form large filamentarystructures, which are often connected with the loci of HI shells. Thederived mean size of the stellar associations in this survey was foundto agree with the average size found previously by other authors, forstellar associations in different galaxies. This common size of about 80pc might represent a universal scale for the star formation process,whereas the parameter correlations of the detected loose systems supportthe distinction between open clusters and stellar associations.
| The relation between radio flux density and ionising ultra-violet flux for HII regions and supernova remnants in the Large Magellanic Cloud
We present a comparison between the Parkes radio surveys (Filipovic etal. 1995) and Vacuum Ultra-Violet (VUV) surveys (Smith et al. 1987) ofthe Large Magellanic Clouds (LMC). We have found 72 sources in common inthe LMC which are known HII regions (52) and supernova remnants (SNRs)(19). Some of these radio sources are associated with two or more UVstellar associations. A comparison of the radio flux densities andionising UV flux for HII regions shows a very good correlation, asexpected from theory. Many of the Magellanic Clouds (MCs) SNRs areembedded in HII regions, so there is also a relation between radio andUV which we attribute to the surrounding HII regions.
| A New Spectral Classification System for the Earliest O Stars: Definition of Type O2
High-quality, blue-violet spectroscopic data are collected for 24 starsthat have been classified as type O3 and that display the hallmark N IVand N V lines. A new member of the class is presented; it is the secondknown in the Cyg OB2 association, and only the second in the northernhemisphere. New digital data are also presented for several of the otherstars. Although the data are inhomogeneous, the uniform plots bysubcategory reveal some interesting new relationships. Several issuesconcerning the classification of the hottest O-type spectra arediscussed, and new digital data are presented for the five original O3dwarfs in the Carina Nebula, in which the N IV, N V features are veryweak or absent. New spectral types O2 and O3.5 are introduced here assteps toward resolving these issues. The relationship between thederived absolute visual magnitudes and the spectroscopic luminosityclasses of the O2-O3 stars shows more scatter than at later O types, atleast partly because some overluminous dwarfs are unresolved multiplesystems, and some close binary systems of relatively low luminosity andmass emulate O3 supergiant spectra. However, it also appears that thebehavior of He II λ4686, the primary luminosity criterion atlater O types, responds to other phenomena in addition to luminosity atspectral types O2-O3. There is evidence that these spectral types maycorrespond to an immediate pre-WN phase, with a correspondingly largerange of luminosities and masses. A complete census of spectraclassified into the original O3 subcategories considered here (notincluding intermediate O3/WN types or O3 dwarfs without N IV, N Vfeatures) totals 45 stars; 34 of them belong to the Large MagellanicCloud and 20 of the latter to 30 Doradus.
| The Effects of Dust in Simple Environments: Large Magellanic Cloud H II Regions
We investigate the effects of dust on Large Magellanic Cloud (LMC)H II region spectral energy distributions usingarcminute-resolution far-ultraviolet (FUV), Hα, far-infrared(FIR), and radio images. Widely used indicators of the amount of lightlost to dust (attenuation) at Hα and in the FUV correlate witheach other, although often with substantial scatter. There are twointeresting systematic discrepancies: First, Hα attenuationsestimated from the Balmer decrement are lower than those estimated fromthe Hα-to-thermal radio luminosity ratio. Our data, at this stage,cannot unambiguously identify the source of this discrepancy. Second,the attenuation at 1500 Å and the UV spectral slope, β,correlate, although the slope and scatter are substantially differentfrom the correlation first derived for starbursting galaxies by Calzettiet al. Combining our result with those of Meurer et al. forultraluminous infrared galaxies and Calzetti et al. for starburstinggalaxies, we conclude that no single relation between β and 1500Å attenuation is applicable to all star-forming systems.
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| H I Shells in the Large Magellanic Cloud
A recent high-resolution H I survey of the Large Magellanic Cloud (LMC)shows that the structure of the neutral atomic interstellar gas isdominated by numerous shells and holes, as well as complex filamentaryand spiral-type structure. We present an up-to-date catalog of candidateH I supergiant and giant shells in the LMC. The candidates are visuallyselected from the H I data cube using selection and classificationcriteria that are described. Twenty-three supergiant shells, defined asthose regions whose extent is much larger than the H I scale height, arecataloged; 103 giant shells (radii less than the scale height of the H Igas) are cataloged. We further classify the H I shells into fivedifferent types, based on the comparison of the H I with theirassociated Hα emission. For this purpose, we obtained newwide-field Hα images of the LMC with a CCD camera mounted on a 16inch (0.41 m) telescope at Siding Spring Observatory. The pixel size of20" and the field of view of 12 deg are well matched to the H I survey.The size distribution of H I shells follows a crude power law,N(logR)~R-1.5. For constant energy input to the H I shellsand a constant shell creation rate, a shell luminosity spectrum of theform φ(L)~L-β, where β=1.75+/-0.2, isobtained. This agrees well with the observed H II region luminosityspectrum for the LMC of Kennicut, Edgar, & Hodge, which hasβ=1.75+/-0.15. H I shells containing H II regions and OBassociations seem to expand more rapidly than those without, providingdirect evidence for substantial input of mechanical energy from regionsof star formation.
| Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of theLarge Magellanic Cloud (reported simultaneously by Cole et al.), we haveused a three-dimensional Monte Carlo radiation transfer code toinvestigate the escape of near-ultraviolet photons from young stellarassociations embedded within a disk of dusty material (i.e., a galaxy).As photons propagate through the disk, they may be scattered or absorbedby dust. Scattered photons are polarized and tracked until they escapethe dust layer, allowing them to be observed; absorbed photons heat thedust, which radiates isotropically in the far-infrared where the galaxyis optically thin. The code produces four output images: near-UV andfar-IR flux, and near-UV images in the linear Stokes parameters Q and U.From these images we construct simulated UV polarization maps of theLMC. We use these maps to place constraints on the star+dust geometry ofthe LMC and the optical properties of its dust grains. By tuning themodel input parameters to produce maps that match the observedpolarization maps, we derive information about the inclination of theLMC disk to the plane of the sky and about the scattering phase functiong. We compute a grid of models with i=28 deg, 36 deg, and 45 deg, andg=0.64, 0.70, 0.77, 0.83, and 0.90. The model that best reproduces theobserved polarization maps has i=36 deg+2-5 andg~0.7. Because of the low signal-to-noise in the data, we cannot placefirm constraints on the value of g. The highly inclined models do notmatch the observed centrosymmetric polarization patterns around brightOB associations or the distribution of polarization values. Our modelsapproximately reproduce the observed ultraviolet photopolarimetry of thewestern side of the LMC; however, the output images depend on many inputparameters and are nonunique. We discuss some of the limitations of themodels and outline future steps to be taken; our models make somepredictions regarding the polarization properties of diffuse lightacross the rest of the LMC.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. II. The Large Magellanic Cloud
A survey of extended objects in the Large Magellanic Cloud was carriedout on the ESO/SERC R and J Sky Survey Atlases, checking entries inprevious catalogs and searching for new objects. The census provided6659 objects including star clusters, emission-free associations, andobjects related to emission nebulae. Each of these classes containsthree subclasses with intermediate properties, which are used to infertotal populations. The survey includes cross identifications amongcatalogs, and we present 3246 new objects. We provide accuratepositions, classification, and homogeneous measurements of sizes andposition angles, as well as information on cluster pairs andhierarchical relation for superimposed objects. This unification andenlargement of catalogs is important for future searches of fainter andsmaller new objects. We discuss the angular and size distributions ofthe objects of the different classes. The angular distributions show twooff-centered systems with different inclinations, suggesting that theLMC disk is warped. The present catalog together with its previouscounterpart for the SMC and the inter-Cloud region provide a totalpopulation of 7847 extended objects in the Magellanic System. Theangular distribution of the ensemble reveals important clues on theinteraction between the LMC and SMC.
| LMC HII region luminosities versus observed ionizing stars
We use the stellar census of OB associations in the Large MagellanicCloud (LMC) to predict the H-alpha luminosities of the host HII regions,based on results from stellar atmosphere models. These values arecompared to the observed HII region luminosities, yielding an estimatefor the mean fraction of H-ionizing photons that escape the localnebulae in this sample. We formally estimate that, overall, 0% to 51% ofthe ionizing radiation escapes the local HII regions and is available toionize the warm, ionized medium in the LMC. We find both nebulae thatappear to be density-bounded, and ones that appear to beradiation-bounded.
| Extinction of H II regions in the Large Magellanic Cloud
The extinction properties of H II regions in the Large Magellanic Cloudare investigated using radio continuum data obtained from the MolongloObservatory Synthesis Telescope, digitized and calibrated H-alpha data,and published Balmer decrement measurements. The resultingextinction-color excess diagram suggests that (1) most H II regions inthe Magellanic Clouds have similar extinction properties to the Galacticones, (2) all imaginable gas/dust configurations are possible, and (3)the extinction of some highly reddened H II region cores originatesexternally in cocoon shells. The puzzle of different extinction-colorexcess ratios of Galactic and extragalactic H II regions is explained asbeing due to the different populations of observed samples rather thanany intrinsic differences. The extinction of the observed Galactic H IIregions produced by foreground dust overwhelms the internal extinction,while the situation in the observed extragalactic H II regions is justthe opposite.
| Comparison of H II region luminosities with observed stellar ionizing sources in the Large Magellanic Cloud
We estimate the total predicted Lyman continuum emission rates of OBassociations for which the complete census of O star spectral typesexists. The results are compared to the observed H-alpha luminosities ofthe host H II regions. We find evidence for substantial leakage ofionizing photons from some H II regions, while others appear to beradiation-bounded. We estimate that overall for the LMC, 0-51 percent ofthe ionizing radiation escapes the local nebulae, and would be availableto ionize the diffuse, warm, ionized medium (WIM) in that galaxy. Thisrange of values is consistent with the observed 35 percent fraction ofH-alpha luminosity emitted by the WIM in the LMC, as well as thecorresponding fractions observed in other nearby galaxies. It istherefore possible that photoionization by O stars is indeed thedominant ionization mechanism for the WIM.
| UBV Photometry of OB Associations within Superbubbles of the Large Magellanic Cloud
This work presents UBV photometry of the stellar populations associatedwith seven superbubble nebulae and five classical H II regions in theLarge Magellanic Cloud. Although the nebular morphology of thesuperbubbles appears to be substantially evolved compared to theclassical nebulae, the color-magnitude diagrams do not reveal anynoticeable correlation between the resident stellar population andnebular morphology. The photometry presented here will be used in aforthcoming paper to examine further the stellar content and dynamics ofthese superbubbles.
| The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..914V&db_key=AST
| Morphology and stellar content of complexes in the LMC.
Three LMC stellar aggregates and two LMC stellar complexes locatedinside the constellations Shapley I, IV, IX, and X have been examined inorder to study their morphology and properties. Star counts wereperformed on excellent quality film copies of direct plates taken withthe 1.2m U.K. Schmidt telescope. They have been used for derivingisodensity contour mapping of the four studied regions. Low dispersionobjective prism plates taken with the same telescope were also used toclassify the spectra of the stars down to M_V_~0.0mag. Combination ofthe two sets of data was used to define the boundaries of these regions,their age, the density and the spatial distribution of their OB stars.It is therefore found that the bright and massive OB type stars are thepredominant stellar component of the four studied regions. They areembedded in a fainter and less massive stellar component within theboundaries of a region, revealed by the isopleths, where the limitingdetection magnitude is down to M_V_~1.5mag. Thus it appears that thestellar content of the complexes and aggregates is made not only by thestars as massive as ~40Msun_, but they also contain lowermass stars, at least ~3Msun_. The spatial distribution ofearly type stars (down to M_V_~0.0mag) shows a gradient which reveals aregion coinciding with the one, defined by the isopleths, where fainterstars are also located. For the near future, we plan to study whetherthe gradient of the radial distribution of the early type starsrepresents the mass distribution of the molecular cloud from which thesestellar structures are formed, or it is due to sequential star formationprocess and/or expansion because of the high stellar winds of the verymassive stars.
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| Extinction characteristics of giant HII regions - Star-forming complexes in the galaxies M33, LMC, and NGC 2403
The discrepancies between the extinction of gas emission and that of thestarlight in giant HII regions, star-forming complexes in the galaxiesM33, LMC, and NGC 2403, were empirically investigated. The extinctionvalues were determined for 30 stars in eight associations in M33. Anempirical relation between the extinction of starlight and that of thegas emission in giant HII regions, star-forming complexes in thegalaxies under study, was obtained.
| A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.
| A Search for Methanol Masers in the Magellanic Clouds
We report the discovery of a second methanol maser in the LargeMagellanic Cloud and we present the results of synthesis observations ofthis and the methanol maser detected previously. The second discoverywas made using the Australia Telescope National Facility's 64-m Parkesradio telescope during an extensive maser search for 6.6-GHz maseremission from the 5_1_-6_0_ A^+^ transition in both Magellanic Clouds.Spectra were obtained towards 35 HII regions in the Large MagellanicCloud and 13 regions in the Small Magellanic Cloud, and also on a3-arcmin grid over an area 0.3^deg^ square, south of the 30 Doradusnebula. Parkes observations at 12.2 GHz towards the two maser sitesyielded no detectable emission from the 2_0_-3_-1_, E methanoltransition. The results suggest that methanol masers are less abundantin the Magellanic Clouds than in our Galaxy. Observations of the twomasers with the Australia Telescope Compact Array showed one to belocated near the continuum emission peak of the H II region MC18 (N11),while the other appeared to be centred near OH emission on thesouth-eastern boundary of MC23 (N105a).
| Results of the ESO / SEST Key Programme on Co/ in the Magellanic Clouds - Part One - a Survey of Co/ in the Large Magellanic Cloud and the Small Magellanic Cloud
As the first part of the ESO-Swedish SEST Key Programme on CO in theMagellanic Clouds, we have observed ^12^CO J = 1-0 towards 92 positionsin the LMC and 42 positions in the SMC. In the SMC we searched foremission from H II regions, dark clouds and IRAS infrared sources. Thegenerally negative detection rate of non-IRAS sources in the SMC led toan LMC source selection based on the IRAS results. In both galaxies, COwas detected towards the majority of sources observed. We also observed^13^C0 J = 1-0 towards the brighter ^12^CO sources in the LMC (37) andSMC (9). Compared to the strength of CO lines observed in the Milky WayGalaxy with identical linear resolutions, velocity-integrated COemission is weaker by at least a factor of three in the LMC sources andan order of magnitude in the SMC sources. The mean velocity-integratedisotopic intensity ratio I_12_/I_13_ is 12.5 in the LMC and about 15 inthe SMC. Individual ratios range from 8.5 to 20. These isotopicintensity ratios are two to three times higher than those found inGalactic molecular clouds.
| A comparison of far infrared and H-alpha emission of H-II regions in the Magellanic Clouds
From a comparison of the IRAS and smoothed H-alpha maps of theMagellanic Clouds, it was found that H-II regions with core-halostructure usually have higher F(60 microns)/F(H-alpha) ratios andprobably emit more in the far infrared than do extended low-density H-IIregions. This is consistent with the idea that the far infrared emissionis mainly produced by dust within H-II regions.
| Near-infrared spectra and classification diagnostics of Seyfert galaxies
Observational results of a previous spectroscopic survey of Seyfertgalaxies in the near-IR are presented, and the potential for usingemission-line ratios in this spectral region as a classificationdiagnostic tool is examined. Near-IR CCD spectra, which cover thewavelength range of 7000-10,000 at a nominal resolution of about 12 A,of 15 additional Seyferts and two starburst galaxies are obtained.Relative emission-line intensities from these observations, incombination with measurements from previous studies and measurements ofnew, signal-to-noise ratio optical spectra of many of these objects, areused to study the diagnostic diagrams involving forbidden S III 9069,9531/H-alpha, forbidden O II 7320, 7330/H-alpha, forbidden S II 6716,6731/H-alpha, and forbidden O III 5007/H-beta. Comparisons are made inthese diagrams between observational data from the active galaxies andpublished measurements of H II regionlike objects, as well as withpredictions from simple one-component models calculated for the twotypes of objects.
| A Comparison of Far Infrared and Hα Emission of HII Regions in the Magellanic Clouds
Not Available
| Infrared photometry of compact objects in the Magellanic Clouds
The results of JHK (L-prime M) photometry of 34, mostly compact, objectsin the Magellanic Clouds are presented. At JHK, slightly less than halfare dominated by stellar emission or are in fact stars. The planetarynebula N 145 (LMC) was also observed. Comparison with IRAS observationsshows that almost all optically compact nebulae are also compact atlonger infrared wavelengths. IRAS 12-micrometer 'excess' emission isinversely correlated with hot and compact IR emission. Some compactobjects, notably the nebula N 88 (SMC) show a rather extreme blue J - Hcolor which needs further investigation. It is suggested that N 59-IRSand especially N 150 (LMC) are good candidates to search for traces ofrecent star formation. Results on the complexes N 159 (LMC), N 83/N 84,and N 88 (SMC) are briefly discussed.
| The detection of X-ray emission from the OB associations of the Large Magellanic Cloud
A systematic study of the X-ray properties of OB associations in theLarge Magellanic Cloud has been carried out using data from the EinsteinObservatory. An excess of young, X-ray-bright supernova remnants isfound in the vicinity of the associations. In addition, diffuse X-rayemission is detected from over two dozen other associations;luminosities in the 0.16-3.5 keV band range from 2 x 10 to the 34th (thedetection threshold) to 10 to the 36th ergs/s. For several of the moreluminous examples, it is shown that emission from interstellar bubblescreated by the OB stellar winds alone is insufficient to explain theemission. It is concluded that transient heating of the bubble cavitiesby recent supernovae may be required to explain the observed X-rays andthat such a scenario is consistent with the number of X-ray-brightassociations and the expected supernova rate from the young stars theycontain.
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